1. Field of the Invention
This invention relates generally to a reciprocating drive/pump system for use in hydraulic applications, and, in a preferred embodiment, for use in a viscometer, which, generally speaking, is a device for measuring the viscosity of fluids. More particularly, but not by way of limitation, the present invention relates to a bidirectional dual piston syringe pump assembly and its use in a reciprocating capillary viscometer.
2. Background
Syringe pumps or cylinder pumps are found in a wide variety of applications, such as manufacturing process control, medical devices, testing instruments, hydraulic systems, and the like. Accumulators are often used in conjunction with such pumps as storage devices to maintain a relatively constant pressure over changing operating conditions and during transient conditions.
Syringe pumps typically comprise a movable piston housed in a cylinder and a rod connecting the piston to a reciprocating driving source, such as a linear actuator. A fluid is drawn into the pump as the piston is moved away from one end of the cylinder and the fluid is pumped from the cylinder as the piston is driven in the other direction. If the diameter of the cylinder is known and the velocity of the piston is known, then the precise flow rate of the fluid may be determined. A bidirectional syringe pump utilizes both sides of the piston such that, as the piston is moved in a direction, fluid is drawn into a first chamber at one end of the cylinder while fluid already present in the a second chamber is pumped out the other end of the cylinder. When the direction of movement of the piston is reversed, fluid is then pumped from the first end while being simultaneously drawn into the other end.
There are many types of viscosity measuring devices. One predominant method for measuring viscosity employs a rotating device that measures the mechanical drag between a rotating member and a stationary member. Rotating viscometers work quite well for many fluids, however, some fluids have properties that actually cause them to climb out of the shear gap, thereby yielding erroneous results, when subjected to testing with such devices. In addition, complications arise with rotating devices when the fluid under test must be under pressure or heated. For example, if the fluid under test must also be under pressure, a rotating seal is required which could impede measuring sensitivity. These types of devices require frequent maintenance of delicate components.
Another type of viscosity measuring device is the reciprocating capillary viscometer. A capillary viscometer is used to determine the viscosity of a fluid by forcing a fluid through a fixed length of capillary tubing with a known diameter, at a known velocity, while measuring the differential pressure across the tube. Mathematical equations which are well known in the art allow calculation of the viscosity of the fluid under test as well as other rheological properties. By periodically reversing the direction of flow through the tubing in a reciprocating manner, continuous measurements may be made on a relatively small sample of fluid.
In the past, reciprocating capillary viscometers have comprised a high pressure chromatography pump, a water or oil bath, capillary tubing, two or more piston accumulators, motor actuated 4-way valves, and a data acquisition and control system. The pump can only move fluid in one direction, so hydraulic fluid is pumped from a reservoir into a first piston accumulator which isolates the fluid under test from the hydraulic fluid. The hydraulic fluid moves the piston thereby forcing the test fluid from the first piston accumulator, through the capillary tube, and into a second piston accumulator. The 4-way valve is used to reciprocatingly apply the hydraulic pressure to each of the piston accumulators. In this way, the fluid under test is first directed to flow from the first accumulator, through the capillary tube in one direction, and into the second accumulator, and then, upon reversal of the 4-way valve, from the second accumulator, back through the capillary tubing in the opposite direction, and back into the first accumulator.
To maintain a uniform temperature of the test fluid, a reciprocating capillary viscometer will typically employ a water or oil bath. While effective, this technique has proven to be large, to be messy, and to have slow heat-up and cool-down performance.
Reciprocating capillary viscometers have typically been large, complex devices requiring frequent and difficult maintenance.
A further limitation of reciprocating capillary viscometers has been the difficulty in loading the fluid under test and cleaning the device between different test fluids.
It is thus an object of the present invention to provide a reciprocating capillary viscometer which is relatively small and less complex, when compared to past devices.
It is a further object of the present invention to provide a method for easily loading the fluid under test and cleaning the viscometer between test fluids.
It is another object of the present invention to provide an improved method for effectively controlling the temperature of the test fluid while measuring rheological properties of such fluid.
It is a still further object of the present invention to provide a bidirectional fluid pump with an integral means for maintaining the fluid pressure within the viscometer.
The present invention provides a reciprocating capillary viscometer which satisfies the needs and alleviates the problems discussed above. The inventive apparatus preferably comprises; a bidirectional syringe pump; a heated mandrel on which is wound a first tube, a capillary tube and a second tube; a differential pressure transducer for measuring the pressure across the capillary tube; and valves for filling and cleaning the apparatus. When the piston of the pump is moved in a first direction, the test fluid is forced from the first chamber of the pump, through the first tube, through the capillary tube in a first direction, through the second tube, and into the second chamber of the syringe pump. When the pump is moved in the other direction, the test fluid is pumped from the second chamber, through the second tube, through the capillary tube in a second direction, through the first tube and back into the first chamber. A differential pressure gauge is used to measure the pressure drop across the capillary tube while the fluid is moving at a known velocity. Mathematical formulas which are well known in the art allow rheological properties of the test fluid to be determined.
An important aspect of the present invention is the bidirectional syringe pump which has potential applicability in other applications, such as hydraulics, process control, and instrumentation. The syringe pump comprises a cylinder with a driven piston and a floating piston contained therein. The floating piston provides a third pump chamber between the driven piston and the floating piston. This chamber may be pressurized by an external pressure source such that the operating pressure of the hydraulic system may be easily maintained at a pressure established by an external pressure source. When used with the inventive viscometer, the chamber between the driven piston and the floating piston of the inventive bidirectional syringe pump additionally provides a cushion to allow for the expansion or contraction of the test fluid, as the test fluid is heated or cooled, without producing an adverse change in pressure of the test fluid. The floating piston enables the closed loop reciprocating hydraulic circuit to operate at relatively high system pressure with a relatively low horsepower motor driving the syringe pump.
In another aspect of the present invention, there is provided a bidirectional syringe pump driven by a precision, ball-screw actuator.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.